High voltage cathode heater



March 26, 1940. R. M. SQM RS 2,194,678

HIGH VOLTAGE OATHODE HEATER Filed Feb. 10, 1937 .nawal Sou cE .0-

lNVEN-TOR Rza/zard M. Somera BYv &

ORNEY Patented Mar. 26, 1940 UNITED STATES HIGH VOLTAGE CATHODE HEATER Richard M. Sc'incrs, West Orange, N. J., assignorfi to Thomas A. Edison,

Incorporated, West Orange, N. J .,a corporation of New Jersey Application February lO, 1937, Serial No. 124,989

8 Claims. (01. 250-275) This invention relates to heaters for discharge device cathodes, and more particularly to heaters upon which is impressed a relatively high voltage.

The invention especially concerns and is particularly described in connection with cathode heatem for gaseous discharge devices, the termhigh voltage conveniently denoting a' voltage drop across the heater of which at least peakyalues are appreciably in excess of the ionization poten tial of the gaseous filling, and typically of the order of several times that potential. By the term gaseous filling I intend, of course, to refer not merely to gases but also to vapors -such, for example, as mercury vapor-or mixtures of gases and vapors.

In the use of high voltage heaters in gaseous devices I have found it first necessary to provide enclosing means which well shield the heater from the principal space within the device, and thus from discharge between a portion of the heater and some one ofthe intendedly active elements of the device. In some instances the heater is placed within a hollow cathode (of which only the external surface is activated), and the interior cathode surface then becomes the wall of the enclosing means; while in other cases the heater (usually on a ceramic or like tube) is disposed around the cathode, and a distinct surrounding elementthen provides the enclosing means wall. But, although the enclosure so described is a first necessity, it is not the only important specification; a high voltage heater made simply in conformity with it is found in use quickly to burn out. One special form of heater has at times been used with satisfactory life-0ne wherein the heater is deeply imbedded ininsulating material, substantially impervious to material disintegrating from the heater, which insulating material, for example, may extend into contact with some surrounding element.

subject to disintegration, are characterized by extremely sluggish action and are useless for many purposes which require reasonably rapid Another object of the invention isthe provision of a high voltage cathode-heater system which will function for a substantial and satisfactory cording to my invention;

But heaters of this type, which may be considered as substantially unmeans close to the heater, physically and in relife in a gaseous discharge device withoutarcbacks involving the heater circuit. i

Another object is the provision of a generally: improved high voltage cathode heater system,

especially for a gaseousdischarge device. 1 '5 Still another objectis the provision of an irnproved electrode structure comprising a cathode and internal heater therefor. g

Other and allied objects will more fully appear from the following description and the appended 10 l claims.

In the descriptionreference is had to the ac companying drawing, of which:

' Figure 1 is an enlarged, partial cross-sectional view of a dischargedevice, illustrating a form'of l5 cathode-heater system therein according to my, invention;

Figure 2 is across-sectional the line 22 of Figure 1;

Figure 3 is a fractional view illustrating a mod- 20 ification of Figure .1 in respect of a detail;

Figure 4 is an enlarged, partial cross-sectional view of a discharge device, illustrating an'aIterna tive form of cathode-heater systemtherein acview taken along Figure 5 is a cross-sectional View takenlalong the line 5-5 of Figure 4;

Figure. 6 is a further enlarged fractionalview of a portion of the structure of Figure 5, illustrating a modification thereof; and

Figure '7 is a view similar to Figure 6 but illusw trating a further modification of the structure ofFigure 5.

When a high voltage heater which is subject toddisintegration is placedwithin the cathode,

it is customary to arrange the combination ,so that the desired cathode temperature will be de l veloped by a minimum heater power dissipation. 1 This is accomplished by quitelclose cathode-toheater spacing, which makes for mostei ficient heat transfer. To do thisis a natural iinclina tion, for if a heater with a given power; consumption have a poor life, changes which would entail an increase of that consumption naturally appear to bein the Wrong direction.

,And when a similar high voltage heater has been placed outside the cathode, it has again been customary to keep the wall of the enclosing spect of temperature, This has been done not only for such heater energy conservation as it may afford, but also in the effort to prevent, arcs from one heater point to another by virtue of a narrow, confined arcing space. It has even been of the order of the mean free path length of electrons under operating conditions within the enclosure.

I have found, however, that these practices overlook the fundamental cause of the poor heater life and lie in precisely the wrong direction. It is not simple thermal disintegration in itself which entails the poor heater life, as it can be shown that the operating temperature and mass of the heater may readily be arranged so that thousands of hours life would be obtained were thermal disintegration alone to be considered. And while it is true that the immediate cause of heater failure is cross-arcing involving the heater circuit, this cross-arcing is not in itself a natural tendency in View of the normally high cathode fall of the heater. I find that the immediately destructive. cross-arcing is in turn predicated upon'the gradual deposition, on the member on which the heater is wound or other insulation in contact with various portions of the heater, of metal which has disintegrated or evaporated from the heater; that this deposition leads rapidly to the development of heater-shorting currents which, as they develop and/or as they burn out the thin deposit in spots, produce minute arcs; and that these minute arcs in turn wipe out, at their spots of occurrence, the normally high cathode fall of the heater-which high fall results from the heater material, for example tungsten, and its lack of oxidecoating and is the factor which ultimately must be relied on to prevent cross-arcing. When this high cathode fall is so wiped out, cross-arcing of course ensues and the heater is destroyed. (Alsothe partial shorting of the heater raises the current in the balance to abnormally high values-reducing the average cathode fall and thus further facilitating arc-back productiom and even in some cases increasing the thermal disintegration, ordinarily negligible in itself, to directly destructive values.) 50 I find the cumulative material deposition on the heater-contacting insulation as a fundamental action whose elimination would solve the problem at hand.

This deposition I findto be dependent on the parameters of the combination of heater, heatercontacting insulation, and the wall of the heaterthose parameters. of which advantage may be taken completely to eliminate any significant accumulation of the deposit in question. This relationship is most easily demonstrated as follows:

Let Eh represent the rate of evaporation of heater material from the heater, and let E1 represent the rate at which evaporation of heater ma-- terial tends to take place from the heater-contacting insulation (i. e., at which it would take place if there were always on that insulation a reserve quantity of that material); these factors depend on heater and insulation temperatures, and are therefore fairly constant for varying positions of the heater-surrounding element.

Let Dw represent the rate of deposition of heater material on the wall of the heater-enclosing means, and let D1 represent the rate at which deposition of heater material tends to take place on the heater-contacting insulation (i. e., at which it would take place thereon if there were no is in equilibrium, so Eh+Ei=Dw+Di.

The relative values of Dw and D1 (whose sum may warrantably be considered approximately constant) depend on temperatures and positions of the wall and heater-contacting insulation; temperature conditions alone tend to make Dw infinitely large compared to D1, position conditions alone tend exact oppositely. But not only do the temperatureconditions usually preponderate in controlling the relative values, to make Dw somewhat the larger, but also any rearrangement affecting both temperature and position tendsto be much more rapidly effective on Dw and Di through temperature. So removal of the wall of the enclosing means to greater .distances from the heater rapidly increases Dw and decreases Di. Writing the above expression in the form Eh=D20+ (Di-Ei) it will immediately be seen that such removal of the wall will rapidly decrease the parenthesis term, and that at some critical value of distance that term will be renderedzero. But that term (when positive) represents the net rate of accumulation on the insulation-wherefore that accumulation may be eliminated by a spacing between heater and wall which equals a certain critical value. And since a negative value of the parenthesis term represents only a tendency, impossible of fulfilment,

to depletion of a non-existent supply of heater material on the insulation, it follows that any accumulation is also avoided by over-critical values of the spacing in question. Of course, with many times over-critical spacing other influences than those specifically considered above may acquire an opportunity, lacking at nearly critical spacings, to affect the material deposition, and I therefore prefer to employ a spacing of between one and two times the critical value.

Alternatively, these considerations may broadly be stated in terms of temperature: that the heating of the wall of the enclosing means must be only to a temperature which is sufiiciently lower that that of the heater-contacting insulation to attract away from the latter substantially all disintegrated heater material.

By the application of the principles so set forth and explained, I have succeeded readily in constructing high voltage heater systemsboth with heater inside and with heater outside the cathodes-which have functioned satisfactorily in gaseous devices for thousands of hours. Precise critical spacing values can obviously not be generalized; they are influenced in individual cases by various factors-such, for example, as the heat reflection, radiation and transmission characteristics of the wall of the enclosing means. But with the knowledge that at least a critical spacing must be. used, and the knowledge that the growth of an accumulation of disintegrated material on the heater support or other heater- 7 contacting insulation indicates an under-critical have satisfactorily constructed and operated, and

to set forth certain secondary considerations helpful in the construction of desirable systems.

Figures 1 and 2 illustrate a cathode of the hollow type, with inside heater; this cathode is disposed at the end of a gaseous discharge device having the envelope i and re-entrant stem 2.

The cathode proper is in the form of a cylinder '3, with" its inner end 30: (i. e., that facing toward the center of the discharge device) crimped or otherwise constricted to fit closely against and electrically contact with the-inner end of a central supporting lead-wire 4. By way of example, the cathode cylinder 3 may be formed of thin nickel (e. g., of the order of .0025" thick), may have a diameter of and a length of about 1%", and may be exteriorly coated with a highly emissive compound such as a compound of alkaline earth oxides. The outer end of the cathode cylinder 3 may be fitted quite tightly over the inner end of a ceramic supporting tube 5, the outer end of which may fit within a tubular inward extension 2a of the stem 2.

The lead-in wire 6 (for example of some .04 diameter) may be surrounded from the stem 2 to near its inner extremity by a ceramic insulating rod 6, centrally pierced to fit the lead-in wire and for example of an external diameter of the order of .09"; rod forms a support for the heater. The heater, designated as I, may consist simply of a suitable length and size of tungsten wire wound about the rod 6 throughout a major central portion of the length of the cathode 3. For example, for operation on a voltage of the order of 90 to 120 volts (D. C. or

r. m. s. A. C.) the heater may be wound of about a 6" coiled length tungsten wire spiral formed, with about 330 turns per inch on an .011" mandrel, of 8 milligram per 200 millimeter wire. The inner end of the heater 1 may be connected, for example through the short loop in; of more rugged wire such as .007" molybdenum, to the lead-in wire dqthe' outer end of the heater may beconnected, asthrough the short loop 1b of molybdenum wire, to a lead-in wire 8- which passes through the stem 2 into the annular space between the ceramic tube 5 and the rod 6. Both the lead-in wires 4 and 8 from the seal inwardly may be of nickel, but are preferably of more refractory material such as molybdenum; their portions within and outside of the seal, however, are desirably of tungsternif a hard glassis employed, and the designations of those portions have accordingly been furnished with the distinguishing additional designation a.

It will be seen that the enclosure about the heater-formed by the surfacezb of the stem 2, the ceramic tube 5, and the cathode cylinder 3, of which latter the main portion forms the wall of the enclosurequite isolates the heater from the principal space within the discharge device.

Spacedly surrounding the cathode cylinder 3, and for example of about twice its diameter, may

be provided, if desired, a thin nickel heat-conserving cylinder such as I have illustrated, in

ment of the instant invention in its broader aspects.

Cathode-heater systems constructed as so illustrated and described, with the approximate di- .mensioning as set forth above by way of example, have been operated by me successfully in gaseous discharge devices for lives or" over 1000 hours with continuously applied voltages across the heater of 90 volts, and of substantially the It will be understood, however, that have burned out'at the end of such periods have given evidence that their final failure was due to extraneous factors and not that fundamental one with the elimination. of which my invention is primarily concerned. The spacing between heater and cathode (which latter, in this case, is the enclosurewall)approximately or about .070-hasbeen found to be for this cathode-heater system slightly greater than the critical spacing, (and of course materially less than twice the latter). The presence or absence of the cylinder 9 has an efiect on the value of critical spacing, since it afiects the cooling facility of the cathode cylinder 3 (and hence the cathode cylinder temperature for given heater dissipation or the heater dissipation required for given cathode temperature) but, in View among other things, of the open-ended nature of the particular cylinder 9 illustrated, this effect in this instance is relatively slight and within the margin of excess over critical spacing embodied in this particular cathode-heater system. As a generality, it will be understood that a determination of critical spacing for a given cathode-heater system is to be made while holding essentially constant the cathode cooling facilities and either the heater dissipation or the cathode temperature, as the requirements of the particular use may dictate.

The structure of Figure 1 has been arranged for community of potential between the cathode and one extremity of the heater, the source l2 of heater current (typical voltages for which have been mentioned above) being connected between the external portions 411 and 5c of the common lead-in wire 4 and the other heater lead-in wire 8, respectively. It is, however, equally satis factory to bring out a separate lead-in wire for the cathode cylinder 3, and to utilize the leadin wire 4 solely for one of the heater connections. In this case, the structure near the inner end of the cathode may be re-arrangcd as shown in Figure 3, wherein the wire 4 terminates a little short of the closed cathodecylinder extremity 3a, but still supports this end of the cathode .by virtue of an annular ceramic bushing 93 fitted tightly within the cathode cylinder and about the wire at the extremity of the latter.

and 5 illustrate a cathode with surrounding heater. with the wall of an enclosure for the'heater in turn surrounding the latter: the cathode is disposed at the end of gaseous discharge device having the envelope 25 and. by of example, the outwardly extending seal 22. The cathode proper is shown in the form of a small cup the interior of which facesinwardly' of the discharge device and is coated with highly compound. such for example as above mentioned. The cathode is supported on and electrically connected to a wire (for example of molybdenum), which. leads in the direction of the 22 to connect with a lead-in wire 2d; this wire may be of nickel or preferably molybdenum. except inthe portion 2M within and external of the which is preferably of tungsten or other sealing wire. Closely surrounding the cathode. but several times its axial length. is theceramic tube 25: this tube may for example be of approximately .270" outside diameter and long, with .030" thick wall. On this tube, almost throughout its length. is

wound the heater 2'! hereinafter discussed.

Spacedly surrounding theheater ,iscthe cylindrical enclosure wall 33, for example of thin nickel, (e. g., .005" thick) and of a diameter of about I The heater enclosure is completed, for heater isolation from the principal space within the device, and the several elements held in proper relationship, by means of the annularly flanged inner and outer end members or washers 34 and $5: the outer periphery of member 34 is flanged outwardly of the device to fit over the cylindrical wall 33, and the inner periphery is similarly flanged to fit within the ceramic tube 28; the outer periphery of member 35 is flanged outwardly cf the device to fit within the cylindrical wall 33, and the inner periphery is inwardly flanged tofit within the tube 25. The central aperture 34a of member 34 is, of course, left open for passage of the intended discharge therethrough, but the like aperture of member 35 is closed by a disc 36 placed thereover; the cathode wire 23a passes through a central hole in the disc 36 within a ceramic tube 31'. A support for the enclosure may be provided by the lead-in wire 33 welded thereto, of nickel except for a tungsten portion 380. within and external to the seal 22; this may form a connection to the enclosure independent of the other elements of the system and, while it may be left disconnected externally, it may if desired be employed for special purposes such for example as discussed above in connection with the cylinder 9 of Figure 1.

The heater Z'l, for operation on 45 to 60 volts, may for example consist of a suitable length and size of tungsten wire, and may by way of example comprise 120 cm. of tungsten wire which weighs about 24 milligrams per 200 millimeters. Its inper extremity may be connected, through the wire Zla preferably of molybdenum, to the leadin wire 26; in its passage through the length of the enclosure and through the members 35 and 36 the wire 21a may be protected by the ceramic tube 39. The outer heater extremity may be connected, as through the loop 215 of molybdenum, to the lead-in wire 28, desirably of nickel excepting in its tungsten section 28a. Because of the exposure of this non-cathode potential lead-in wire to the principal space within the device which would otherwise occur, it is surrounded from within the enclosure to the stem by the ceramic tube 25; and any possible arcing to the wire 28 at the junction of stem and tube 25 is prevented by an inward tubular extension 22a of the stem about the ceramic tube 25. The elements 25 and 22a are in this functional respect analogous to the elements 5 and 2a of Figure 1; and this preventive function is frequently an important refinement in the successful operation of the high voltage cathode heater.

Cathode-heater systems constructed as so illustrated and described in and with reference to Figures 4 and 5, with approximate dimensioning as set forth above by way of example, has been operated by me successfully in gaseous discharge devices for lives of many thousand hours with continuously applied voltages across the heater of 45 volts (D. C. or r. m. s. A. (3.); and similar stems but with suitably revised heater wire constants, heating the cathode to just as high a temperature, have been similarly successfully operated for lives of a few thousand hours with continuously applied voltage across the heater of volts, and of substantially the same order on volts. The spacing between heater and enclosure wallapproximately or .1l"--is again one and a minor fraction times the critical spacing with these parameters.

As in Figure 1, the system of Figures 4-5 may of course be arranged for non-coincidence of cathode potential with either heater extremity; but in this case the wire 21a should be as fully shielded from within the stem to within the heater enclosure as is the lead-in wire 28.

It is to be understood that the insulating means which contacts with various heater portions, and on which material deposition is to be avoided, is not limited to a ceramic or like tube on which the heater is wound. It may instead for example comprise a coating of insulating material between or between and on top of the turns of the heater-being in the latter case of course of material which is appreciably porous or previous to disintegrated heater material, and in this respect differing from the impervious material sometimes employed to envelop heaters as hereinabo-ve mentioned. Thus in Figures 6 and 7, which may be taken for example as enlarged showings of a small portion of Figure 5, I have shown respectively any insulating material 4! between the turns of the heater, and a porous insulating material 42 between and on top of the heater turns. Such insulating material is frequently employed to advantage, for example in rendering the heater mechanically more stable, and the avoidance of deposit of disintegrated heater material is secured similarly to the avoidance of deposit on a simple supporting member.

While the life figures above mentioned concern total time of application of the mentioned voltages across the heater, the cathode of Figures 4-5 will be recognized as one well adapted to be employed, as well as in other cases, in are discharge devices wherein after a short pre-heating period the supply of current to the heater is reduced, either finitely or to zero, and the arc itself relied on for continued cathode heating. In such service the ratio of total service during heating-up periods to total service during normal running periods is high, and it is then especially desirable to consider the temporary conditions during a heating-up period. The enclosure wall, of small mass and of low specific heat and subject principally to heating by radiation, may rise rapidly in temperature; on the other hand the heater-supporting member or the like, of larger mass and higher specific heat and subject largely to conduction heating, may rise much more slowly in temperature. Thus temporarily the heater support, or heater-contacting insulation, may be cooler than normal relative to the enclosure wall, and deposition thereon may temporarily occur; and if the heater running periods at normal temperatures are too short, they may not be sufficient to wipe out these temporarily occurring depositions. Therefore it is desirable to use a slightly greater over-critical spacing in these cases, as well as to keep as low as practicable the mass of the heater support and other heater-contacting insulation.

It will be seen in both of the illustrated structures that I have preferred to keep the heater enclosure a very intact one, with very negligible atmospheric passages thereinto; the fit of cathode 3 over ceramic tube 5 of Figure 1, and of ceramic tubes 25 and '39 through end member 34 of Figure 5, etc., being for example as snug as prac ticable in each instance.

While I have described my inventionwith ref erence to specific embodiments thereof, I do not intend to be limited by the details of thoseem bodiments, which are by-nature typical rather than comprehensive; rather I intend herebelow to claim my invention asbroadly asthe state'of the art will permit.

' capable of operation for a substantial life, with,-

out destruction by such disintegration alone, at a voltage of which at least peak values are appreciably in excess of the lowest ionizing poteno tial of said filling, insulating means in contact with various portions of said heater, and an. enclosure about said heater isolating the same from the principal space within said device; said enclosure having a wall spaced from said heater by 5 a direct path which is substantially pervious to disintegrated heater material and which has a length greater than the mean free path length of electrons in said filling and sufficient so that said insulating means remains substantially free of such material during said operation.

2. In an electrical system including a discharge device having a gaseous filling: a cathode-heater system within said device comprising an electrical heater subject to thermal disintegration but capable of operation for a substantial life, without destruction by such disintegration alone, at a voltage of which at least peak values are appreciably in excess of the lowest ionizing potential of said filling, insulating means in contact with various portions of said heater, and an enclosure about said heater isolating the same from the principal space within said device; said enclosure having a wall spaced from said heater by a direct path which is substantially pervious to disintegrated heater material and which has a length greater than the mean free path length of electrons in said filling and at least as great as the critical wall spacing at whichthe rates of attempted deposition and vaporization of disintegrated heater material, on and from said insulating means, balance each other during said operation.

3. In an electrical system including a discharge device having a gaseous filling: a cathode-heater system within said device comprising an electrical heater subject to thermal disintegration but capable of operation for a substantial life, without destruction by such disintegration alone, at a voltage of which at least peak values are appreciably in excess of the lowest ionizing potential of said filling, insulating means in contact with various portions of said heater, and an enclosure about said heater isolating the same from i the principal space within said device; said enclosure having a wall spaced from said heater by a direct path which is substantially pervious to disintegrated heater material and which has a length of more than the mean free path length of electrons in said filling and of between one and two times the'critical wall spacing at which the rates of attempted deposition and vaporization of disintegrated heater material, on and from said insulating means, balance each other during said operation.

1. In an electrical system including a discharge device having a gaseous filling: a cathode-heater system within said device comprising an electrical heater subject to thermal disintegration but capable of operation for a substantial life, without destruction by such disintegration alone, at a voltage of which at least peak values areappreciably in excess of the lowest ionizing potential of said filling, insulating means in contact with various portions of said heater and heated thereby, and an enclosure about said heater isolating the same from/the principal T space within said device; said enclosure having a wall spaced from said heater by a direct'path which is substantially pervious to disintegrated. heater material and which isl'on'ge'r than the mean free path length of electrons in said filling, and heated by said heater only to a temperature sufliciently lower than that of said insulating means to attractaway from said insulating means substantially all disintegrated heater material tending to deposit thereon during said operation.

5. In an electrical system including a discharge device having a gaseous filling: a cathodeheater system Within said device comprising an electrical heater subject to thermal disintegration but capable of operation for a substantial life, without destruction by such disintegration alone, at a voltage of which at least peak values are in excess of 40 volts, insulating means in contact with various portions of said heater, and.

an enclosure about said heater isolating the same from the principal space within said device; said enclosure having a wall spaced from said heater by a direct path which is substantially pervious to disintegrated heater material and which has a length greater than the mean free path length of electrons in said filling and sufficient so that said insulating means remains substantially free of such material during said operation.

6. In an electrical system including a discharge device having, a gaseous filling: a cathodeheater system within said device comprising an electrical heater subject to thermal disintegration but capable of operation for a substantial life, Without destruction by such disintegration alone, at a voltage of which at least peak values are in excess of 40 volts, insulating means in contact with various portions of said heater, and an enclosure about said heater isolating the same from the principal space within said device; said enclosure having a Wall spaced from said heater by a direct path which is substantially pervious to disintegrated heater material and which has a length greater than the mean free path length of electrons in said filling and at least as great as the critical wall spacing at which the rates of attempted deposition and vaporization of disintegrated heater material, on and from said insulating means, balance each other during said operation.

'7. In an electrical system including a discharge device having a gaseous filling: a cathodeheater system within said device comprising an electrical heater subject to thermal disintegration but capable of operation for a substantial life, without destruction by such disintegration alone, at a voltage of which at least peak values are in excess of 40 volts, insulating means in contact with various portions of said heater, and an enclosure about said heater isolating the same from the principal space within said device; said enclosure having a wall spaced from said heater by a direct path which is substantially pervious to disintegrated heater material and which has a length of more than the mean free path length of electrons in said filling and of between one and two times the critical wall spacing at which the rates of attempted deposition and vaporization of disintegrated heater material, on and from said insulating means, balance each other during said operation.

8. In an electrical system including a discharge device having a gaseous filling: a cathodeheater system within said device comprising an electrical heater subject to thermal disintegration but capable of operation for a substantial life, without destruction by such disintegration alone, at a voltage of which at least peak values are in excess of 40 volts, insulating means in contact with various portions of said heater and heated thereby, and an enclosure about said heater isolating the same from the principal space within said device; said enclosure having a wall spaced from said heater by a direct path which is substantially pervious to disintegrated heater material and which is longer than the mean free path length of electrons in said filling, and heated by said heater only to a temperature sufiiciently lower than that of said insulating means to attract away from said insulating means substantially all disintegrated heater material tending to deposit thereon during said operation.

RICHARD M. SOMERS. 

